[AIGTWY-4573] Parse the managed coding-agent config

[david-siqi-liu]

Parse the managed CodingAgentConfig and add the version and watermark helpers on their own, ahead of any launch or configure change, so the data-shape change reviews in isolation. Nothing here runs on the launch path yet; the child PR wires it in.

Changes:

• Model the wire CodingAgentConfig as frozen dataclasses (CodingAgentConfig, EnabledAgent/AgentConfig, AgentModels, the NamesOrLocation selector, SpendTiers) parsed via from_wire and projected to the internal shape via to_internal, so the interface shape is legible instead of scattered dict reads. The model source is the server's oneof: exactly one of model_provider_service / unity_catalog_location / model_services.
• mcp_servers and skills each parse to a names-or-location selector: an absolute names list of UC FQNs, or a unity_catalog_location (a catalog.schema) whose contents are discovered, exactly one.
• Greenfield, so no back-compat: the deprecated wire fields the proto reserves are not read (custom_headers, the model_config oneof, tracing_config, tags), and neither are the reserved top-level display_name or the removed tracing table.
• managed_config: add managed_config_is_newer / managed_update_time, a spec_version forward-compat gate (a newer or malformed spec_version keeps the last-known-good cache), and the UCODE_MANAGED_CONFIG_STUB local-file read for pre-server testing.
• state: add the applied-version watermark (get/set_applied_managed_update_time) the launch gate reads.
• managed_setup: serialize and validate the same shape (names are 3-part FQNs, location a 2-part catalog.schema, exactly one set), so serialize then normalize round-trips.
• databricks: the managed-config update mask sends the mutable, non-reserved paths (default_agent, enabled_agents, mcp_servers, skills, spend_tiers); rename the user-facing "Unity AI Gateway" strings to "Unity Gateway".

This pull request and its description were written by Isaac.

[lilly-luo]

could you add a @DataClass that represents the overall API? it's difficult to grok the shape right now. the dataclass will also help with request / response validation, it's easy to make an error and use the wrong interface